This invention relates to tin compounds useful as catalysts. More specifically, the invention relates to amine-modified tin compounds which can catalyze the hydrolysis of latent amine curing agents for a wide variety of amine curable monomers and prepolymers useful as adhesives, sealants and coatings.
Many monomers and prepolymers can be cured or cross-linked by amines. Examples of such amine-curable materials include the various acrylates, epoxides, urethanes, anhydrides and the like. Formulations of these curable materials are often made up as one component systems in which the monomer or prepolymer is admixed with the amine curing agent. To provide for adequate shelf life, the curing agent must be in a deactivated or latent form until the formulation is ready for use. The curing agent is then activated via exposure to moisture which causes a hydrolysis reaction to take place to thereby regenerate the amine. A representative example of this process is the moisture induced hydrolysis of a ketimine to form the corresponding amine and ketone (R.sup.1, R.sup.2 and R.sup.3 representing well known substituent groups): EQU R.sup.1 R.sup.2 C.dbd.NR.sup.3 +H.sub.2 O.fwdarw.R.sup.1 R.sup.2 C.dbd.O+H.sub.2 NR.sup.3
The regeneration of the amine by hydrolysis is not instantaneous, however, and since the rate of cure is a function of available amine curing agent, the cure time is dependent on the rate of hydrolysis.
For many applications, it is highly desirable to have a rapid initial or partial moisture cure to provide "green strength". Green strength results from polymerization occurring at ambient temperatures, and an adhesive or sealant with high green strength has high initial adhesion or tack. Furthermore, sufficient green strength will hold an adhesive bond in place during subsequent processing and final curing stages.
The prior art discloses generally the use of tin compounds in polymerizations employing amine-based curing agents, but does not teach the use of such compounds to catalyze hydrolysis reactions. U.S. Pat. No. 4,616,066 discloses in Col. 8, line 5, the use of dibutyltin dilaurate (DBTDL) in a polymer system which includes an epoxy resin and a ketimine made into an aqueous emulsion for electrodeposition. However, the DBTDL serves not as a hydrolysis catalyst for the ketimine, but rather as a curing agent in the later baking step. U.S. Pat. No. 4,713,481 discloses in Col. 7, line 29, the use of DBTDL in a polymerization process which includes the reaction of a ketone-blocked ethylenediamine and an isocyanate-terminated polyurethane. The DBTDL is used to catalyze formation of the polyurethane.
U.S. Pat. No. 4,711,944 relates to humidity resistant coatings of copolymers of t-butyl acrylate and polyepoxides. In Col. 5, line 50, there is disclosed the use of stannous octoate or DBTDL as a catalyst for the reaction of epoxy groups to crosslink hydroxyl, carboxyl or amide functions on the polyacrylate. The stannous octoate and DBTDL are not used as hydrolysis catalysts for latent amines.
The prior art has disclosed non-tin based compounds for accelerating moisture cure. In U.S. Pat. No. 4,861,832, amines are listed in Col. 5, lines 20-30 as co-curing agents in a reaction involving epoxy resin and a ketimine. Acids are also known as hydrolysis catalysts for latent amines as taught by U.S. Pat. No. 4,816,542 at Col. 3, lines 53-55, where they catalyze the cure of polyurethane prepolymers.
Thus, while the prior art recognizes the need for catalysis of various reactions involving the moisture curing of monomers and prepolymers, there has heretofore been no use of a tin catalyst for such application.